The present invention relates to a method and an apparatus for rapidly charging a battery to a predetermined level corresponding to the voltage of the battery.
Heretofore, it has been known to prevent any overcharge of a rechargeable battery by detecting a voltage drop (-.DELTA.V) which occurs when the voltage of the battery reaches a peak value upon completion of the charging and then by breaking a charging current. This will be referred to as a "-.DELTA.V charge control method". Such a method is disclosed in U.S. Pat. No. 3,938,021 to Kosmin and Japanese Patent Publication (Examined) No. 18177/85.
While the -.DELTA.V charge control method is advantageous in that the battery can reliably be fully charged and the battery can effectively be prevented from being broken or damaged due to overcharging, problems are involved.
Generally, it is preferable that the value of -.DELTA.V be in the range of 10 to 20 mV per a one-cell battery for a battery voltage of 1.2 V; 20 to 40 mV in the case of a two-cell battery for a battery voltage of 2.4 V; and 100 to 200 mV in the case of a ten-cell battery for a battery voltage of 12 V. When the value of -.DELTA.V is set to a value in the range of 100 to 200 mV, while an optimum charging can be attained for the ten-cell battery, the two-cell battery results in overcharge. On the other hand, when the value of -.DELTA.V is set to a value in the range of 20 to 40 mV, while an optimum charging can be attained for the two-cell battery, the ten-cell battery results in undercharge. That is, with a fixed value of -.DELTA.V, some batteries are either overcharged or undercharged depending on the battery voltage.
With another charging method in which a charge termination signal is issued while monitoring the battery voltage with a microcomputer, the following problems are encountered.
If the battery voltage is applied to the microcomputer via resistors for a voltage division and also via an A/D converter, the value of -.DELTA.V is small, say about 10 to 20 mV per a one-cell battery, so that the value of -.DELTA.V cannot be discriminated if the battery voltage is further dropped due to the voltage division when applying to the A/D converter. For example, when charging a battery of two to ten cells (2.4 to 12 V), if the battery voltage is divided to one-third by the resistors, a voltage applied to an 8-bit converter is 0.8 V for the two-cell battery, and 4 V for the ten-cell battery. Assuming that the output of the A/D converter is [FF].sub.H ([ ].sub.H indicates a hexidecimal notation, and hereinafter subscript H is omitted) when the input thereto is 5 V, the output thereof is [CC] when 4 V, and [28] when 0.8 V.
The resolution power with the 8-bit converter is: ##EQU1## That is, the resolution power is 19.6 mV per one bit. The value of -.DELTA.V of the two-cell battery is 20 to 40 mV and the voltage applied to the A/D converter is 6.7 to 13.3 mV, so that -.DELTA.V is not detectable. Therefore, it is necessary to increase the value of -.DELTA.V of the two-cell battery, alternatively it is necessary to increase the bit number of the A/D converter.